I have a 3 block also, I also have a 68 block.
There's absolutely no difference except a different looking area where the oil sensor goes on the back of the block and thicker motor mount bosses. If there was something really special about the 440-3 don't you think everyone would want them? Besides the reason people get them for being cheap. I'll have to go out and measure the main webbing I believe it also got a little bit bigger but it's only to make up for strength taken away from the cheaper (less nickel) metal used for casting. I know this because it is a visual difference in what comes out of a bore while boring it. Later blocks just grey cast iron, early blocks had shiny blued shavings and also less dust during boring.